Lead acid batteries are known to include at least one positive current collector, at least one negative current collector, and an electrolytic solution including, for example, sulfuric acid (H2SO4) and distilled water. Ordinarily, both the positive and negative current collectors in a lead acid battery are configured as lead grid-like plates. The role of these lead current collectors is to transfer electric current to and from the battery terminals during the discharge and charging processes. Storage and release of electrical energy in lead acid batteries is enabled by chemical reactions that occur in a paste disposed on the current collectors. A notable limitation to the durability of lead acid batteries is corrosion of the lead material of the positive current collector.
The rate of corrosion of the positive current collector is a major factor in determining the life of the lead acid battery. Once the sulfuric acid electrolyte is added to the battery and the battery is charged, each positive current collector of the battery is continually subjected to corrosion due to its exposure to sulfuric acid and to the anodic potentials of the positive collector. One of the most damaging effects of this corrosion is volume expansion. Particularly, as the positive current collector corrodes, lead dioxide is formed from the lead source metal of the current collector. This lead dioxide corrosion product has a greater volume than the lead source material consumed to create the lead dioxide. Corrosion of the lead source material and the ensuing increase in volume of the lead dioxide corrosion product is known as volume expansion.
Volume expansion induces mechanical stresses on the current collector that deform and stretch the current collector. At a total volume increase of the current collector of approximately 4% to 7%, the current collector may fracture. As a result, battery capacity drops, and eventually, the battery will reach the end of its service life. Additionally, at advanced stages of corrosion, internal shorting within the current collector and rupture of the cell case can occur. Both of these corrosion effects may lead to failure of one or more of the cells within the battery.
One method of extending the service life of a lead acid battery is to increase the corrosion resistance of the positive current collector. Several methods have been proposed for inhibiting the corrosion process in lead acid batteries. Because carbon does not oxidize at the temperatures at which lead acid batteries generally operate, some of these methods have involved using carbon in various forms to slow or prevent the detrimental corrosion process. For example, U.S. Pat. No. 5,512,390 (hereinafter the '390 patent) discloses a lead acid battery that includes current collectors made from graphite plates instead of lead. The graphite plates have sufficient conductivity to function as current collectors, and they are more corrosion resistant than lead. Substituting graphite plates for the lead current collectors may, therefore, lengthen the life of a lead acid battery.
While the battery of the '390 patent may potentially offer a lengthened service life as a result of reduced corrosion of the positive current collector, the graphite plates of the '390 patent are problematic. For example, the graphite plates of the '390 patent are dense, flat sheets of material each having a relatively small amount of surface area. Unlike lead electrode plates of a conventional lead acid battery, which are generally patterned into a grid-like structure to increase the available surface area of the plates, the graphite plates of the '390 patent are smooth sheets with no patterning. In lead acid batteries, an increase in surface area of the current collector may increase the specific energy of the battery and, therefore, may translate into improved battery performance. More surface area on the current collectors may also lead to a reduction in the time required for charging and discharging of the battery. The relatively small surface area of the graphite plates of the '390 patent results in poorly performing batteries that have slow charging speeds.
Additionally, the graphite plates of the '390 patent lack the toughness of lead current collectors. The dense graphite plates of the '390 patent are brittle and may fracture when subjected to physical shock or vibration. Such physical shock and vibration commonly occur in vehicular applications, for example. Any fracturing of the graphite plates would lead to the same problems caused by volume expansion of ordinary lead current collectors. Therefore, despite offering an increased resistance to corrosion compared to conventional lead current collectors, the brittle nature of the graphite plates of the '390 patent could actually result in battery service lives shorter than those possible through use of ordinary lead current collectors.